Atomic Models and Subatomic Particles

The Three Subatomic Particles

An atom is the smallest unit of an element that retains the element's properties. It is built from three subatomic particles. You must know the charge and relative mass of each cold.

The proton carries a +1 charge and a mass of about 1 atomic mass unit (u). The neutron is neutral (charge 0) with a mass of about 1 u. The electron carries a –1 charge and a mass roughly 1/1836 of a proton — so small the exam treats it as having essentially no mass.

Where the mass and charge live

Protons and neutrons sit together in the nucleus, a tiny, dense, positively charged core. Because electrons have negligible mass, nearly all of an atom's mass is in the nucleus. Electrons occupy the large volume around the nucleus, so most of an atom's volume is empty space.

Atomic Number and Mass Number

Two numbers define any atom, and the exam uses them in nearly every atomic-structure cluster.

• Atomic number (Z): the number of protons in the nucleus. It defines the element — every carbon atom has 6 protons; nothing with 6 protons is anything but carbon.
• Mass number (A): the total number of protons plus neutrons. It is always a whole number for a specific atom.

Key relationships you will use repeatedly:

• Number of protons = atomic number (Z)
• Number of neutrons = mass number − atomic number (A − Z)
• In a neutral atom, number of electrons = number of protons

Worked example: An atom of ³⁹K (potassium-39) has Z = 19. So it has 19 protons, 19 electrons (neutral), and 39 − 19 = 20 neutrons.

Ions and charge

When an atom gains or loses electrons it becomes an ion. Losing electrons leaves a net positive charge (a cation); gaining electrons gives a net negative charge (an anion). Protons never change in chemical processes — only electrons move. A Ca²⁺ ion has 20 protons but only 18 electrons.

How the Model Was Built

The Regents exam expects you to connect each scientist to the evidence that changed the model. This is classic Engineering/Technology and Nature-of-Science content.

• John Dalton (early 1800s): atoms are tiny, solid, indivisible spheres; all atoms of an element are identical (the "billiard ball" model).
• J.J. Thomson (1897): discovered the electron using cathode rays; proposed the "plum pudding" model — negative electrons embedded in a positive sphere.
• Ernest Rutherford (1911): the gold-foil experiment — most alpha particles passed straight through thin gold foil, but a few bounced sharply back. This proved the atom is mostly empty space with a small, dense, positively charged nucleus.
• Niels Bohr (1913): electrons travel in fixed energy levels (shells) around the nucleus; explained the bright-line spectra of hydrogen.
• Wave-mechanical (modern) model: electrons exist in orbitals — regions of probable location forming an "electron cloud." We cannot know an electron's exact path.

The gold-foil experiment in detail

Rutherford fired positively charged alpha particles at gold foil. The two key observations and conclusions:

1. Most particles passed straight through → the atom is mostly empty space.
2. A few were deflected at large angles or bounced back → there is a small, dense, positive region (the nucleus) that repels the positive alpha particles.

This is a model of how science self-corrects: a single experiment overturned Thomson's plum-pudding picture and forced a new model. The Regents nature-of-science strand expects you to pair the evidence with the conclusion, not just memorize names.

Reading the Nucleus from Notation

A quick reference for translating any atom into particle counts. Use the Periodic Table to find Z, then apply the relationships.

Worked check: For a sulfide ion, S²⁻, sulfur has Z = 16. Protons = 16, electrons = 16 + 2 = 18 (it gained 2 electrons to get the 2− charge), and neutrons depend on which isotope but never change the charge.

Common Exam Traps

• Confusing atomic number with mass number. Atomic number = protons only; mass number = protons + neutrons. If a question gives mass number, you must subtract to get neutrons.
• Changing protons for an ion. Ions form by gaining or losing electrons, never protons. Changing protons would change the element.
• Crediting Bohr with the nucleus. The nucleus came from Rutherford's gold-foil experiment; Bohr added the energy-level idea on top of it.
• Assuming electrons add mass. Because the electron's mass is negligible, gaining or losing electrons does not noticeably change an atom's mass — but it does change its charge.
• Forgetting neutral means equal counts. A neutral atom has equal protons and electrons; the moment those differ, you have an ion with a net charge.